The use of silicon based capacitive transducers as microphones is well known in the art. Typically, such microphones consist of four elements: a fixed backplate; a highly compliant, moveable diaphragm (which together form the two plates of a variable air-gap capacitor); a voltage bias source and a buffer.
The two mechanical elements, the backplate and diaphragm, are typically formed on a single silicon substrate using a combination of surface and bulk micromachining well known in the art. One of these two elements is generally formed to be planar with the surface of the supporting silicon wafer. The other element, while itself generally planar, is supported several microns above the first element by posts or sidewalls, hence the term raised microstructure.
In general, the positioning of the two elements with respect to each other, affects the performance of the entire device. Intrinsic stresses in the thin films comprising the raised microstructure cause the structure to deflect out of the design position. In a microphone in particular, variations in the gap between the diaphragm and backplate affect the microphone sensitivity, noise, and over pressure response.
Many other factors also affect the manufacture, structure, composition and overall design of the microphone. Such problems are more fully discussed and addressed in U.S. Pat. No. 5,408,731 to Berggvist; U.S. Pat. No. 5,490,220 to Loeppert, and U.S. Pat. No. 5,870,482 to Loeppert.
In the specific example of the design of a microphone backplate as a raised microstructure, the goal is to create a stiff element at a precise position relative to the diaphragm. One method to achieve this is to form the backplate using a silicon nitride thin film deposited over a shaped silicon oxide sacrificial layer which serves to establish the desired separation. This sacrificial layer is later removed through well known etch processes, leaving the raised backplate. Intrinsic tensile stress in the silicon nitride backplate will cause it to deflect out of position. Compressive stress is always avoided as it causes the structure to buckle.
FIG. 1 depicts one such raised microstructure 10 of the prior art. After the oxide is removed leaving the raised microstructure 10, an intrinsic tension will be present within the plate 12. This tension T results from the manufacturing process as well as from the difference between the coefficient of expansion of the material of the raised microstructure 10 and the supporting wafer 16. As shown, the tension T is directed radially outwards. The tension T intrinsic in the plate 12 will result in a moment as shown by arrow M about the base 18 of sidewall 14. This moment M results in a tendency of the plate 12 to deflect towards the wafer 16 in the direction of arrow D. This deflection of plate 12 results in a negative effect on the sensitivity and performance of the microphone.
A number of undesirable means to negate the effects of this intrinsic tension within a thin-film raised microstructure are known in the prior art. Among them are that the composition of the thin film can be adjusted by making it silicon rich to reduce its intrinsic stress levels. However, this technique has its disadvantages. It results in making the thin film less etch resistant to HF acid, increasing the difficulty and expense of manufacture. An additional solution known in the prior art would be to increase the thickness of the sidewall supporting the raised backplate thereby increasing the sidewall's ability to resist the intrinsic tendency of the thin film to deflect. While this sounds acceptable from a geometry point of view, manufacture of a thick sidewall when the raised microstructure is made using thin film deposition is impractical.
It is an object of the present invention to overcome the disadvantages of the prior art with respect to the design of raised microstructures in silicon based devices by negating the undesirable effects of the intrinsic thin film tension inherent in said microstructure.